Fabrication of semiconductor devices is a multi-step process. During processing, chucks are employed to immobilize the semiconductor wafers, despite, for example, gas pressure present at their rear, or back, faces. Such immobilization may occur either by vacuum pumping, mechanically, or electrostatically, and commercially-available chucks suitable for this purpose typically employ these principles of operation. Vacuum pumping is useful only in restricted instances, as it is ineffective when very low process chamber pressures are required.
Primarily because they are relatively inexpensive, mechanical clamps have been used widely in the semiconductor industry. Mechanical clamps physically engage the peripheries of the wafers, increasing the risk of contact damage and shielding the clamped areas from treatment or processing. This engagement may also produce debris (i.e. particles) that negatively affect wafer process yields. Mechanical clamps additionally are incapable of securing central portions of the wafers, resulting in undesirable bowing of these unsupported regions.
Electrostatic chucks, by contrast, use Coulombic forces to secure the entire rear face of a semiconductor wafer. As employed, these chucks are of two general types: monopolar and bi- (or multi-) polar. Monopolar electrostatic chucks, effectively functioning as one plate of a parallel-plate capacitor, include a thin dielectric layer coating and a single electrode supported by a base. A wafer, acting as the other capacitive plate, may then be placed over the dielectric. As a voltage is applied between the wafer and electrode, the wafer is attracted to the electrode and flattened against the adjacent dielectric material.
Multipolar electrostatic chucks typically include two or more electrodes themselves held at different electrical potentials. As a result, no net charge transfer to the wafer need occur. Existing bipolar electrostatic chucks are relatively difficult and expensive to construct, however, and may compel adjustment or balancing of the voltages present at the electrodes during processing of the wafer.
U.S. Pat. No. 4,384,918 to Abe, incorporated herein in its entirety by this reference, provides equations for the attractive force generated using electrostatic chucks. For the monopolar chuck, the attractive force "F" may be calculated as follows: ##EQU1## where .epsilon.=the dielectric constant of the dielectric
As a consequence of these mathematical relationships, numerous efforts to decrease the thickness of the dielectric layer have been made. U.S. Pat. No. 4,480,284 to Tojo, et al., also incorporated herein in its entirety by this reference, mentions several such efforts, including using an adhesive agent to bond a mica, polyester, or barium titanate dielectric layer to an electrode disc and making the dielectric layer by an anodic treatment of metallic material. As discussed in the Tojo, et al. patent, however, the dielectric layers formed using these processes typically lack uniform thicknesses and flat surfaces and, in some cases, may crack over time. This unreliability is magnified when coupled with the expense associated with these complicated processes, reducing the commercial value of the processing technology.
The Tojo, et al. patent itself purports to disclose a method of forming a more reliable dielectric layer for use with an electrostatic chuck. According to the patent, aluminum oxide (i.e. alumina), titanium dioxide, or barium titanate (or mixtures thereof) is flame-sprayed on an electrode plate to create the dielectric layer. Because the rough surface generated using the flame spray process lacks sufficient dielectric strength, the layer is impregnated with plastic material. To reduce warping and cracking, the dielectric layer and electrode have substantially the same coefficient of thermal expansion.